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IUPAC name: see Chemistry
Synonyms: Cialis®
Formula: C22H19N3O4

 Uses: Erectile Dysfunction

 Properties: PDE-5 inhibitor

 Hazards: cardiovascular risks

Mass (g/mol): CAS #:

Tadalafil, commonly known by the trade name Cialis®, is a drug used to treat erectile dysfunction. It is a selective phosphodiesterase type-5 (PDE-5) inhibitor that is believed to function in the same way that sildenafil (Viagra®) and vardenafil (Levitra®) work, despite significant structural differences.

Mechanism of action

(CC) Image: David E. Volk

By competitively binding to PDE-5 enzymes in smooth muscle and therefore inhibiting the binding of cGMP to PDE-5, the degradation of cGMP is reduced resulting in elevated levels of cGMP in the corpus cavernosum and its supply vessels. The elevated cGMP levels relax the smooth muscles, dilate the corporeal sinusoids and increase blood flow enabling an erection. cGMP levels are normally increased during stimulation by the release of nitric oxide in the corpus cavernosum. The nitric oxide activates guanylate cyclase, an enyme, which produces cGMP. Thus, tadalafil does not enhance the normal mechanism, namely increased synthesis of cGMP, but rather reduces its degradation.


The chemical designation for tadalafil is pyrazino[1´,2´:1,6]pyrido[3,4-b]indole-1,4-dione, 6-(1,3-benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-, (6R,12aR)-. It is a crystalline solid that is practically insoluble in water and very slightly soluble in ethanol.

Drug interactions

Because tadalafil has vasodilator properties that result in decreased blood pressure, the combined use of tadalafil with other vasodilators, such as alpha-blockers, must be done cautiously. Patients with a history of heart attacks, strokes, arrythmia, hypertension, retinitis pigmentosa or currently on bosentan therapy should also be cautious.

External links

The most up-to-date information about this and other drugs can be found at the following sites. The most up-to-date information about Tadalafil and other drugs can be found at the following sites.


J. D. Corbin and S. H. Sharron. "Molecular Biology and Pharmacology of PDE-5-Inhibitor Therapy for Erectile Dysfunction". J. Androl. 24: S38-S41.